> Sounds like you need two controllers for sure. If you try to write 
> them
> in parallel without the full two controller setup, how will you know
> when one fails?

The application is for photos. I'm a keen amateur photographer and in 
some cases I'm extremely keen not to lose photos. And, the system 
would be saleable if it worked well. [Build it before I do (and I 
probably wont) and I'll buy one :-) ].

If the system is good enough to write at least one good photo of the 
pair with a suitably low error rate then the loss of an occasional A 
or B copy is acceptable as long as both A & B are lost with only very 
low probability. Say 1:1000 for A or B and 1:100,000 for A & B copies 
of a single pghoto would be very acceptable. Better would be better.

What I am trying to avoid is the loss of a whole card full of data, as 
has happened to me enough times so far as to make me nervous when I'm 
doing critical photo sessions. Sometimes the card is recoverable by 
formatting and somtimes its dead. Losing eg the middle  half of a 
wedding service or similar would be 'unfortunate'. What I do now is 
carry a number of CF cards and alternate amongst them whenever there 
is a 'gap in the traffic'. That way, if one card of say 4 fails, you 
lose a number of time stripes and not a whole block. I have a laptop 
setup to suck data when time allows. Next stage is to write to an 
external drive, check the external and internal copies against the CF 
card and only then delete it.



        Russell McMahon

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